From 109b7ca47ec0a0c293b17bd2be3874623d633539 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Mon, 23 Mar 2026 07:32:27 +0800
Subject: [PATCH] Workaround for MSVC's `constexpr` array allocation bug for
 `vector`

---
 stl/inc/vector                                    | 4 ++++
 stl/inc/xmemory                                   | 9 +++++++++
 tests/std/tests/P1004R2_constexpr_vector/test.cpp | 4 ----
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/stl/inc/vector b/stl/inc/vector
index cc54d2137ec..c0e318c287b 100644
--- a/stl/inc/vector
+++ b/stl/inc/vector
@@ -1656,7 +1656,11 @@ private:
             _Newvec = _Allocate_at_least_helper(_Al, _Newcapacity);
         } else {
             _STL_INTERNAL_STATIC_ASSERT(_Policy == _Reallocation_policy::_Exactly);
+#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11060638
+            _Newvec = _Al.allocate(_Newcapacity) + decltype(pointer{} - pointer{}){0};
+#else // ^^^ workaround / no workaround vvv
             _Newvec = _Al.allocate(_Newcapacity);
+#endif // ^^^ no workaround ^^^
         }
 
         _Allocation_guard<_Alty> _Guard{_Al, _Newvec, _Newcapacity};
diff --git a/stl/inc/xmemory b/stl/inc/xmemory
index 6ee39ff8e2b..769ab1d98ae 100644
--- a/stl/inc/xmemory
+++ b/stl/inc/xmemory
@@ -2320,11 +2320,20 @@ _NODISCARD_RAW_PTR_ALLOC _CONSTEXPR20 typename allocator_traits<_Alloc>::pointer
     if constexpr (_Should_allocate_at_least<_Alloc>) {
         auto [_Ptr, _Allocated] = _Al.allocate_at_least(_Count);
         _Count                  = _Allocated;
+#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11060638
+        return _Ptr + decltype(_Ptr - _Ptr){0};
+#else // ^^^ workaround / no workaround vvv
         return _Ptr;
+#endif // ^^^ no workaround ^^^
     } else
 #endif // _HAS_CXX23
     {
+#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11060638
+        using _Pointer = decltype(_Al.allocate(_Count));
+        return _Al.allocate(_Count) + decltype(_Pointer{} - _Pointer{}){0};
+#else // ^^^ workaround / no workaround vvv
         return _Al.allocate(_Count);
+#endif // ^^^ no workaround ^^^
     }
 }
 
diff --git a/tests/std/tests/P1004R2_constexpr_vector/test.cpp b/tests/std/tests/P1004R2_constexpr_vector/test.cpp
index b2702dbf378..c165a2b28a1 100644
--- a/tests/std/tests/P1004R2_constexpr_vector/test.cpp
+++ b/tests/std/tests/P1004R2_constexpr_vector/test.cpp
@@ -679,15 +679,11 @@ constexpr bool test_vector_of_array() {
 #endif // !_HAS_CXX23
     {
         test_vector_of_array_impl<unique_ptr<vector<char>>, 1>();
-#if defined(__clang__) || defined(__EDG__) // TRANSITION, GH-6155 (not yet reported to MSVC)
         test_vector_of_array_impl<unique_ptr<vector<char>>, 42>();
-#endif // ^^^ no workaround ^^^
     }
 
     test_vector_of_array_impl<vector<long>, 1>();
-#if defined(__clang__) || defined(__EDG__) // TRANSITION, GH-6155 (not yet reported to MSVC)
     test_vector_of_array_impl<vector<long>, 42>();
-#endif // ^^^ no workaround ^^^
 
     return true;
 }